This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 10-302994, filed Oct. 23, 1998, the entire contents of which are incorporated herein by reference.
The present invention relates to a microwave plasma generator useful to decompose particularly organic halides, such as freon and trichloromethane, in which the structure of a discharge tube for microwave plasma generation is improved, and a method and system for decomposing organic halides such as freon and trichloromethane.
Large amounts of organic halides such as freon, trichloromethane, and halon, containing fluorine, chlorine, bromine, and the like in molecules, are used in a variety of applications such as refrigerants, solvents, and fire extinguishers. However, these compounds are highly volatile, so many of them are emitted, without being processed, to environments such as the air, soil, and water. Such emitted gases are found to have large influence on environments, e.g., produce carcinogens, destroy the ozonosphere, and cause global warming. Hence, it is necessary to make these organic halides harmless from the viewpoint of environmental protection.
Conventionally reported organic halide processing methods primarily use decomposition reaction at high temperatures. These processing methods are roughly classified into the incineration method and the plasma method.
In the incineration method, volatile organic halides are incinerated together with common wastes such as resins. To burn them in a waste incinerator, corrosion resistance must be improved as a measure against hydrogen chloride which is a strong acid and corrosive. Also, since a combustion temperature different from those for combustion of common resins is set, these organic halides must be separately burned. Furthermore, the exhaust amounts of, e.g., hydrogen chloride and dioxine are strictly restricted. As a consequence, the processing amount cannot be thoughtlessly increased to avoid a primary cause of, e.g., an unstable combustion temperature.
As the plasma method, a method of decomposing a volatile organic halide into carbon dioxide, hydrogen chloride, and hydrogen fluoride by reacting the halide with water vapor in a plasma is known as a dedicated decomposing apparatus for, e.g., freon.
Jpn. Pat. Appln. KOKAI Publication No. 3-222298 has disclosed a microwave plasma trace element analyzer including a double-structure discharge tube shown in FIG. 17. A double-structure discharge tube 201 shown in FIG. 17 is made of quartz and comprises a cylindrical outer tube 202 and a cylindrical inner tube 203. A reaction gas supply pipe 204 is connected to the outer tube 202 in the direction of tangent of this outer tube 202. A tapered large-diameter cylindrical portion 205 of thick section is formed at the end portion of the inner tube 203, thereby narrowing the space between the cylindrical portion 205 of this inner tube 203 and the outer tube 202. The discharge tube 201 is inserted through a microwave cavity 207 having a metal conductor 206 and a rectangular waveguide 208.
A reaction gas 209 is supplied through the supply pipe 204 to the space between the outer tube 202 and the inner tube 203 in the discharge tube 201, where the gap between an antenna and a cavity end plate, or between inner and outer conductors, in the cavity 207 is positioned. This reaction gas 209 is injected from the exit end of this space. At the same time, a carrier gas 210 is supplied into the inner tube 203 and blown off from an injection opening 211 open at the end portion of the inner tube 103, thereby generating a plasma 212 by discharge. In elemental analysis, for example, nitrogen gas or the like is used as the reaction gas 209, and Ar or He is used as the carrier gas 210.
The plasma is ignited by using a Tesla coil placed outside and near the discharge tube 201 at the microwave cavity exit or an ignition coil 214 connected to an ignition power supply 213 such as a neon transformer.
When Ar gas is used as the carrier gas, however, the running cost increases.
If the flow rate of the carrier gas is increased, the generation of a plasma becomes unstable.
Also, since the injection opening 211 of the inner tube 203 for injecting the carrier gas is very small, this portion is readily damaged by, e.g., melting by a plasma.
Additionally, the gas flow path between the inner tube 203 and the outer tube 202 is narrowed by the tapered large-diameter cylindrical portion 205 of thick section formed at the end portion of the inner tube 203. Although this raises the reaction gas injection rate, the plasma 212 flows backward in the space around the cylindrical portion 205. Consequently, the cylindrical portion 205 of the inner tube 203 melts and breaks.
On the other hand, to prevent the contact of a plasma with the wall surface of the discharge tube 201, the gas flow in the discharge tube 201 is usually given a swirling flow effect in the gap between the outer tube 202 and the inner tube 203 by connecting the gas supply pipe 204 to the outer tube 202 in the direction of tangent of this outer tube 202. However, this effect is lost because the thick cylindrical portion 205 of the inner tube 203 narrows the flow path formed by the gap. Consequently, even a slight change in the plasma state causes nonuniform discharge or melts the discharge tube 201.
It is an object of the present invention to provide a microwave plasma generator capable of stably and efficiently generating a plasma.
It is another object of the present invention to provide a method of decomposing an organic halide capable of efficiently decomposing volatile organic halides such as freon and trichloromethane.
It is still another object of the present invention to provide an organic halide decomposing system capable of efficiently decomposing volatile organic halides such as freon and trichloromethane.
A microwave plasma generator according to the present invention comprises:
a rectangular waveguide having a hole to transmit a microwave;
a coaxial microwave cavity connected to the rectangular waveguide by communicating with the hole; and
a discharge tube which is made of a dielectric material, extends through the hole of the rectangular waveguide, and extends through the cavity so as to be coaxial with a central axis of the cavity,
wherein the discharge tube has a double-tube structure including outer and inner tubes,
a sectional area of an annular gap formed between the outer and inner tubes is constant over an entire length of the inner tube, and
an end portion of the inner tube has a gas injection opening having the same diameter as an internal cavity of the inner tube.
Another microwave plasma generator according to the present invention comprises:
a rectangular waveguide having a hole to transmit a microwave;
a cylindrical microwave resonance cavity which is connected to the rectangular waveguide by communicating with the hole and is placed such that a central axis of the resonance cavity aligns with the direction of electric field in the rectangular waveguide; and
a discharge tube which is made of a dielectric material, extends through the hole of the rectangular waveguide, and extends through the resonance cavity so as to be coaxial with the central axis of the cavity,
wherein the discharge tube has a double-tube structure including outer and inner tubes,
a sectional area of an annular gap formed between the outer and inner tubes is constant over an entire length of the inner tube, and
an end portion of the inner tube has a gas injection opening having the same diameter as an internal cavity of the inner tube.
In a method of decomposing an organic halide according to the present invention, which decomposes an organic halide in a plasma by using a microwave plasma generator comprising a rectangular waveguide having a hole to transmit a microwave, a coaxial microwave cavity connected to the rectangular waveguide by communicating with the hole, and a discharge tube which is made of a dielectric material, extends through the hole of the rectangular waveguide, and extends through the cavity so as to be coaxial with a central axis of the cavity, wherein the discharge tube has a double-tube structure including outer and inner tubes, a sectional area of an annular gap formed between the outer and inner tubes is constant over an entire length of the inner tube, and an end portion of the inner tube has a gas injection opening having the same diameter as an internal cavity of the inner tube,
a gas containing the organic halide, water vapor, and air is supplied to the annular gap formed between the outer and inner tubes to generate a plasma, in the outer tube, which extends from the vicinity of the end portion of the inner tube toward an end portion of the outer tube.
In another method of decomposing an organic halide according to the present invention, which decomposes an organic halide in a plasma by using a microwave plasma generator comprising a rectangular waveguide having a hole to transmit a microwave, a coaxial microwave cavity connected to the rectangular waveguide by communicating with the hole, and a discharge tube which is made of a dielectric material, extends through the hole of the rectangular waveguide, and extends through the cavity so as to be coaxial with a central axis of the cavity, wherein the discharge tube has a double-tube structure including outer and inner tubes, a sectional area of an annular gap formed between the outer and inner tubes is constant over an entire length of the inner tube, and an end portion of the inner tube has a gas injection opening having the same diameter as an internal cavity of the inner tube,
the organic halide is supplied into the inner tube, and a gas containing water vapor and air is supplied to the annular gap formed between the outer and inner tubes, thereby generating a plasma, in the outer tube, which extends from the vicinity of the end portion of the inner tube toward an end portion of the outer tube.
In still another method of decomposing an organic halide according to the present invention, which decomposes an organic halide in a plasma by using a microwave plasma generator comprising a rectangular waveguide having a hole to transmit a microwave, a cylindrical microwave resonance cavity connected to the rectangular waveguide by communicating with the hole and placed such that a central axis of the resonance cavity aligns with the direction of electric field in the rectangular waveguide, and a discharge tube which is made of a dielectric material, extends through the hole of the rectangular waveguide, and extends through the resonance cavity so as to be coaxial with the central axis of the cavity, wherein the discharge tube has a double-tube structure including outer and inner tubes, a sectional area of an annular gap formed between the outer and inner tubes is constant over an entire length of the inner tube, and an end portion of the inner tube has a gas injection opening having the same diameter as an internal cavity of the inner tube,
a gas containing the organic halide, water vapor, and air is supplied to the annular gap formed between the outer and inner tubes to generate a plasma, in the outer tube, which extends from the vicinity of the end portion of the inner tube toward an end portion of the outer tube.
The generator may further comprise an ignition electrode installed in the inner tube to generate microwave discharge.
The generator may further comprise an ignition coil installed in the inner tube to generate microwave discharge.
The generator may further comprise a gas supply pipe installed in the gap between the outer and inner tubes along the direction of tangent of the outer tube.
The gas may consist of the organic halide, the water vapor, the air, and argon.
The organic halide may be freon. In accordance with one embodiment, the freon is freon R12. In accordance with another embodiment, the freon is freon R12.
In still another method of decomposing an organic halide according to the present invention, which decomposes an organic halide in a plasma by using a microwave plasma generator comprising a rectangular waveguide having a hole to transmit a microwave, a cylindrical microwave resonance cavity connected to the rectangular waveguide by communicating with the hole and placed such that a central axis of the resonance cavity aligns with the direction of electric field in the rectangular waveguide, and a discharge tube which is made of a dielectric material, extends through the hole of the rectangular waveguide, and extends through the resonance cavity so as to be coaxial with the central axis of the cavity, wherein the discharge tube has a double-tube structure including outer and inner tubes, a sectional area of an annular gap formed between the outer and inner tubes is constant over an entire length of the inner tube, and an end portion of the inner tube has a gas injection opening having the same diameter as an internal cavity of the inner tube,
the organic halide is supplied into the inner tube, and a gas containing water vapor and air is supplied to the annular gap formed between the outer and inner tubes, thereby generating a plasma, in the outer tube, which extends from the vicinity of the end portion of the inner tube toward an end portion of the outer tube.
Another microwave plasma generator according to the present invention comprises:
a rectangular waveguide having a hole to transmit a microwave;
a cylindrical microwave resonance cavity which is connected to the rectangular waveguide by communicating with the hole and is placed such that a central axis of the resonance cavity aligns with the direction of electric field in the rectangular waveguide;
a discharge tube which is made of a dielectric material, extends through the hole of the rectangular waveguide, and extends through the resonance cavity so as to be coaxial with the central axis of the cavity;
a metal conductor which is connected, while being fitted in the discharge tube, to a portion of the discharge tube near the hole, and extends into the resonance cavity through the hole; and
a slidable probe antenna which is interposed between the metal conductor and the discharge tube so as to be slidable in an axial direction of the discharge tube, and extends into the resonance cavity through the hole in the rectangular waveguide.
Still another microwave plasma generator according to the present invention comprises:
a rectangular waveguide having a hole to transmit a microwave;
a cylindrical microwave resonance cavity which is connected to the rectangular waveguide by communicating with the hole, is placed such that a central axis of the resonance cavity aligns with the direction of electric field in the rectangular waveguide, and has an end plate on a bottom portion;
a discharge tube which is made of a dielectric material, extends through the hole of the rectangular waveguide, and extends through the resonance cavity so as to be coaxial with the central axis of the cavity;
a metal conductor which is connected, while being fitted in the discharge tube, to a portion of the discharge tube near the hole, and extends into the resonance cavity through the hole; and
an annular metal conductor which is interposed between the end plate of the resonance cavity and the discharge tube extending through the end plate, and extends from the outside to the inside of the cavity.
Still another microwave plasma generator according to the present invention comprises:
a rectangular waveguide having a hole to transmit a microwave;
a cylindrical microwave resonance cavity which is connected to the rectangular waveguide by communicating with the hole, is placed such that a central axis of the resonance cavity aligns with the direction of electric field in the rectangular waveguide, and has an end plate on a bottom portion;
a discharge tube which is made of a dielectric material, extends through the hole of the rectangular waveguide, and extends through the resonance cavity so as to be coaxial with the central axis of the cavity; and
a metal conductor which is connected, while being fitted in the discharge tube, to a portion of the discharge tube near the hole, and extends into the resonance cavity through the hole,
wherein the end plate of the resonance cavity has a tapered projection, which projects toward the metal conductor, in a portion through which the discharge tube extends.
In a method of decomposing an organic halide according to the present invention, a thermal plasma is generated by irradiating a gas containing an organic halide with a microwave, thereby decomposing the organic halide.
A system for decomposing an organic halide according to the present invention comprises:
a rectangular waveguide having a hole to transmit a microwave;
a coaxial microwave cavity connected to the rectangular waveguide by communicating with the hole;
a reaction tube placed below the cavity;
a discharge tube which is made of a dielectric material, extends through the hole of the rectangular waveguide, and extends into the reaction tube through the cavity so as to be coaxial with the central axis of the cavity;
a metal conductor which is connected, while being fitted in the discharge tube, to a portion of the discharge tube near the hole, and extends into the resonance cavity through the hole;
a vessel into which a lower end of the reaction tube is inserted and which contains an aqueous alkali solution; and
gas supply means for supplying a gas containing an organic halide to the discharge tube.
Another system for decomposing an organic halide according to the present invention comprises:
a rectangular waveguide having a hole to transmit a microwave;
a coaxial microwave cavity connected to the rectangular waveguide by communicating with the hole;
a reaction tube placed below the cavity;
a discharge tube which is made of a dielectric material, extends through the hole of the rectangular waveguide, and extends into the reaction tube through the cavity so as to be coaxial with the central axis of the cavity;
a metal conductor which is connected, while being fitted in the discharge tube, to a portion of the discharge tube near the hole, and extends into the resonance cavity through the hole;
a vessel into which a lower end of the reaction tube is inserted and which contains an aqueous alkali solution;
gas supply means for supplying a gas containing an organic halide to the discharge tube through first piping;
water supply means connected to the first piping through second piping; and
heating means, provided for the second piping, for converting water flowing in the second piping into water vapor.
Still another system for decomposing an organic halide according to the present invention comprises:
a rectangular waveguide having a hole to transmit a microwave;
a coaxial microwave cavity connected to the rectangular waveguide by communicating with the hole;
a reaction tube placed below the cavity;
a discharge tube which is made of a dielectric material, extends through the hole of the rectangular waveguide, and extends into the reaction tube through the cavity so as to be coaxial with the central axis of the cavity;
a metal conductor which is connected, while being fitted in the discharge tube, to a portion of the discharge tube near the hole, and extends into the resonance cavity through the hole;
a vessel into which a lower end of the reaction tube is inserted and which contains an aqueous alkali solution;
gas supply means for supplying a gas containing an organic halide to the discharge tube; and
water vapor spraying means, formed in a circumferential wall of the reaction tube, for spraying water vapor into a thermal plasma generation region near a lower end portion of the discharge tube.
Still another system for decomposing an organic halide according to the present invention comprises:
a rectangular waveguide having a hole to transmit a microwave;
a coaxial microwave cavity connected to the rectangular waveguide by communicating with the hole;
a reaction tube placed below the cavity;
a discharge tube which is made of a dielectric material, extends through the hole of the rectangular waveguide, and extends into the reaction tube through the cavity so as to be coaxial with the central axis of the cavity;
a metal conductor which is connected, while being fitted in the discharge tube, to a portion of the discharge tube near the hole, and extends into the resonance cavity through the hole;
a vessel into which a lower end of the reaction tube is inserted and which contains an aqueous alkali solution;
gas supply means for supplying a gas containing an organic halide to the discharge tube; and
alkali water spraying means, formed in a side wall of the reaction tube, for spraying alkali water into a thermal plasma generation region near a lower end portion of the discharge tube.
Still another system for decomposing an organic halide according to the present invention comprises:
a rectangular waveguide having a hole to transmit a microwave;
a coaxial microwave cavity connected to the rectangular waveguide by communicating with the hole;
a reaction tube placed below the cavity;
a discharge tube which is made of a dielectric material, extends through the hole of the rectangular waveguide, and extends into the reaction tube through the cavity so as to be coaxial with the central axis of the cavity;
a metal conductor which is connected, while being fitted in the discharge tube, to a portion of the discharge tube near the hole, and extends into the resonance cavity through the hole;
a vessel into which a lower end of the reaction tube is inserted and which contains an aqueous alkali solution;
gas supply means for supplying a gas containing an organic halide to the discharge tube; and
heating means, installed in at least one of a position around the reaction tube and a position below the vessel, for vaporizing the aqueous alkali solution in the vessel and introducing water vapor into a thermal plasma generation region near a lower end portion of the discharge tube.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.